1. Technical Field
The present invention relates to an orthogonal frequency division multiplexing access (OFDMA) communication system, and more particularly, to an inter-cell interference mitigation method for overcoming reception performance degradation caused by asynchronous inter-cell interference in a multi-input multi-output (MIMO)-OFDMA system in which a multi-cell environment is considered.
2. Related Art
Next-generation mobile communications such as a 3rd Generation Partnership Project (3GPP) long-term evolution (LTE) or a mobile WiMax of IEEE 802.16 commonly employ communication techniques such as a MIMO antenna technique and an OFDMA scheme.
Most of OFDMA schemes have a problem of inter-cell interference (ICI) caused by overlapping of frequency resources between cells since an inter-cell frequency reuse ratio is one (1). The inter-cell interference is more problematic in a cell structure of an asynchronous type in which synchronization between base stations is not required. As a technique of avoiding the inter-cell interference, techniques such as fractional frequency reuse (FFR) are used, and various techniques for inter-cell interference mitigation are applied at the time of receiver design. The present invention relates to an inter-cell interference mitigation method of a receiver for mitigating the inter-cell interference and a receiver design to which the inter-cell interference mitigation method is applied.
In a multi-path fading channel environment in which mobility is considered, a reception signal is greatly influenced by frequency selective fading caused by a multi-path delay of a channel and a Doppler frequency according to an increase in velocity of a moving body. Particularly, when the velocity of a moving body increases and so the Doppler shift gets deepened, a change of a channel increases even in one frame due to a decrease in a coherence time. Thus, in order to compensate for influence of a channel, it is necessary to accurately estimate a channel frequency response (CFR) on a reception signal.
In addition, when practical cellular environments such as different cell radiuses and a difference in a distance between base stations are considered, signals of terminals (user equipment (UE)) having different symbol timings are received in an overlapping manner. When asynchronous interference signals overlap each other within a Fast Fourier Transform (FFT) window zone of a receiver, the interference signals destroy orthogonality in a frequency signal domain. Inter-cell interference between asynchronous signals and noise signals are estimated using a spatial covariance matrix (SCM).
A representative SCM estimation technique includes a low pass smoothing (LPS) technique that transforms an initial (instantaneous) SCM estimated in a frequency domain into that in a time domain and then applies low-pass type weighting. By this technique, noise included in an uncorrelated component is removed from a time domain characteristic of an observed SCM, and a maximum noise reduction effect can be obtained.
Let us assume that M transmitting antennas and N receiving antennas are installed. In this case, in an inter-cell interference environment, MICI inter-cell interference signals are received in a superimposed manner. Thus, a reception signal in a k-th sub carrier of a j-th OFDM symbol in a frequency domain is represented by Formula 1:Yk,j=Hk,jXk,j+Gk,jUk,j+Wk,j=Hk,jXk,j+Ik,j  Formula 1
Here, a reception signal vector Yk,j, a transmission signal vector Xk,j, a channel coefficient matrix Hk,j for a transmission signal, an asynchronous inter-cell interference signal vector Uk,j, a channel coefficient matrix Gk,j for the asynchronous inter-cell interference signal vector, and a noise vector Wk,j having an average of “0” and a variance of σ2 are represented by Formula 2 in view of the transmitting and receiving antennas. In Formula 1, Ik,j may be represented by the sum of the asynchronous inter-cell interference and noise.
                              [                                                                      Y                                      k                    ,                    j                                    1                                                                                                      Y                                      k                    ,                    j                                    2                                                                                    ⋮                                                                                      Y                                      k                    ,                    j                                    N                                                              ]                =                                            [                                                                                          H                                              k                        ,                        j                                                                    1                        ,                        1                                                                                                                        H                                              k                        ,                        j                                                                    1                        ,                        2                                                                                                  …                                                                              H                                              k                        ,                        j                                                                    1                        ,                        M                                                                                                                                                        H                                              k                        ,                        j                                                                    2                        ,                        1                                                                                                                        H                                              k                        ,                        j                                                                    2                        ,                        2                                                                                                  …                                                                              H                                              k                        ,                        j                                                                    2                        ⁢                        M                                                                                                                                  ⋮                                                        ⋮                                                        ⋱                                                        ⋮                                                                                                              H                                              k                        ,                        j                                                                    N                        ,                        1                                                                                                                        H                                              k                        ,                        j                                                                    N                        ,                        2                                                                                                  …                                                                              H                                              k                        ,                        j                                                                    N                        ,                        M                                                                                                        ]                        ⁡                          [                                                                                          X                                              k                        ,                        j                                            1                                                                                                                                  X                                              k                        ,                        j                                            2                                                                                                            ⋮                                                                                                              X                                              k                        ,                        j                                            M                                                                                  ]                                +                                                                                   [                                                                                                              G                                                      k                            ,                            j                                                                                1                            ,                            1                                                                                                                                                G                                                      k                            ,                            j                                                                                1                            ,                            2                                                                                                                      …                                                                                              G                                                      k                            ,                            j                                                                                1                            ,                                                          M                              ICI                                                                                                                                                                                                                    G                                                      k                            ,                            j                                                                                2                            ,                            1                                                                                                                                                G                                                      k                            ,                            j                                                                                2                            ,                            2                                                                                                                      …                                                                                              G                                                      k                            ,                            j                                                                                2                            ,                                                          M                              ICI                                                                                                                                                                                          ⋮                                                                    ⋮                                                                    ⋱                                                                    ⋮                                                                                                                                      G                                                      k                            ,                            j                                                                                N                            ,                            1                                                                                                                                                G                                                      k                            ,                            j                                                                                N                            ,                            2                                                                                                                      …                                                                                              G                                                      k                            ,                            j                                                                                N                            ,                                                          M                              ICI                                                                                                                                                            ]                                ⁡                                  [                                                                                                              U                                                      k                            ,                            j                                                    1                                                                                                                                                              U                                                      k                            ,                            j                                                    2                                                                                                                                    ⋮                                                                                                                                      U                                                      k                            ,                            j                                                                                M                            ICI                                                                                                                                ]                                            +                              [                                                                                                    W                                                  k                          ,                          j                                                1                                                                                                                                                W                                                  k                          ,                          j                                                2                                                                                                                        ⋮                                                                                                                          W                                                  k                          ,                          j                                                N                                                                                            ]                                                                        Formula        ⁢                                  ⁢        2            
If it is assumed that the CFR (i.e., the channel coefficient matrix) for the reception signal can be perfectly ideally estimated like the conventional SCM estimation technique, in SCM estimation for removing and mitigating the asynchronous inter-cell interference, an initial SCM may be calculated using an OFDM symbol allocated a preamble or a training sequence by Formula 3. Generally, an SCM for the k-th sub carrier of the j-th OFDM symbol is obtained by averaging Ik,j that is the sum of the asynchronous inter-cell interference and noise by P reception OFDM symbols by Formula 4.
                                          R                          k              ,              j                        II                    =                                    (                                                Y                                      k                    ,                    j                                                  -                                                      H                                          k                      ,                      j                                                        ⁢                                      X                                          k                      ,                      j                                                                                  )                        ⁢                                          (                                                      Y                                          k                      ,                      j                                                        -                                                            H                                              k                        ,                        j                                                              ⁢                                          X                                              k                        ,                        j                                                                                            )                            H                                      ,                                  ⁢                  0          ≤          k          ≤                      K            -            1                                              Formula        ⁢                                  ⁢        3                                                                                    R                k                II                            =                            ⁢                              E                ⁡                                  [                                      R                                          k                      ,                      j                                        II                                    ]                                                                                                        =                            ⁢                                                1                  P                                ⁢                                                      lim                                          P                      ->                      ∞                                                        ⁢                                                            ∑                                              j                        =                        0                                                                    P                        -                        1                                                              ⁢                                          [                                              R                                                  k                          ,                          j                                                II                                            ]                                                                                                                                              =                            ⁢                                                1                  P                                ⁢                                                      lim                                          P                      ->                      ∞                                                        ⁢                                                            ∑                                              j                        =                        0                                                                    P                        -                        1                                                              ⁢                                          [                                                                        (                                                                                    Y                                                              k                                ,                                j                                                                                      -                                                                                          H                                                                  k                                  ,                                  j                                                                                            ⁢                                                              X                                                                  k                                  ,                                  j                                                                                                                                              )                                                ⁢                                                                              (                                                                                          Y                                                                  k                                  ,                                  j                                                                                            -                                                                                                H                                                                      k                                    ,                                    j                                                                                                  ⁢                                                                  X                                                                      k                                    ,                                    j                                                                                                                                                        )                                                    H                                                                    ]                                                                                                                                                              =                                ⁢                                                      1                    P                                    ⁢                                                            lim                                              P                        ->                        ∞                                                              ⁢                                                                  ∑                                                  j                          =                          0                                                                          P                          -                          1                                                                    ⁢                                              [                                                                              (                                                          I                                                              k                                ,                                j                                                                                      )                                                    ⁢                                                                                    (                                                              I                                                                  k                                  ,                                  j                                                                                            )                                                        H                                                                          ]                                                                                                        ,                                                          Formula        ⁢                                  ⁢        4            where P represents the number of symbols of the training sequence.
In the case of the multi-path channel in which mobility is considered, a characteristic of an SCM that changes with respect to each OFDM symbol is lost in the process of applying an average as in Formula 4, and thus the performance for removing and mitigating the inter-cell interference that changes every moment degrades. As a method of compensating for this influence, the conventional LPS technique applies time-domain low-pass type weighting in order to obtain a noise reduction effect for the initial SCM instead of taking an average as in Formula 4.
A description will be made below in connection with the LPS-based SCM estimation technique and a structure of a conventional inter-cell interference mitigation receiving apparatus to which the LPS-based SCM estimation technique is applied.
FIG. 1 is a block diagram for explaining a configuration a MIMO-OFDM inter-cell interference mitigation receiving apparatus according to a conventional art.
FIG. 1 shows a MIMO-OFDM receiver having a receiver structure to which an LPS based SCM estimating algorithm is applied.
In N receiving units 110, after a cyclic prefix (CP) is removed from a signal in a time domain, a reference symbol (RS) signal extraction unit 111 separates a preamble or a training sequence allocated to each antenna from a reception signal in a frequency domain that has been subjected to a FFT process, and provides the preamble or the training sequence to an SCM estimation unit 120. The SCM estimation unit 120 receives Hk,j estimated by a channel estimation unit (not shown in FIG. 1) and calculates an initial SCM Rk,jπ by Formula 3. More specifically, FIG. 1 shows that the initial SCM is calculated by an initial SCM estimation unit 121 installed in the SCM estimation unit 120.
At this time, when the initial SCM has a transform relation between the frequency domain and the time domain as in Formula 5, a weighting vector di can be applied by Formula 6 for the sake of the noise reduction effect. That is, instead of taking an average as in Formula 4, time-domain low-pass type weighting is applied to the initial SCM as in Formula 6 in order to obtain the noise reduction effect.
                                          S            k                          (                              n                ,                m                            )                                =                                    R              k              II                        ⁡                          [                              n                ,                m                            ]                                      ,                  where          ⁢                                          ⁢                      {                                                                                                                                                        m                          ,                                                      n                            =                            1                                                    ,                          …                          ⁢                                                                                                          ,                          N                                                                                                                                                                                          k                            =                            0                                                    ,                          …                          ⁢                                                                                                          ,                                                      K                            -                            1                                                                                                                                ⁢                                                                          ⁢                                      s                    i                                          (                                              n                        ,                        m                                            )                                                                      =                                  I                  ⁢                                                                          ⁢                  F                  ⁢                                                                          ⁢                  F                  ⁢                                                                          ⁢                                                            T                      K                                        ⁡                                          [                                              S                        k                                                  (                                                      n                            ,                            m                                                    )                                                                    ]                                                                                  ,                                                          Formula        ⁢                                  ⁢        5            where N represents the number of antennas, and K represents the number of sub carriers.{circumflex over (s)}i(n,m)=di·si(n,m)  Formula 6
That is, the conventional LPS technique was considered to obtain the noise reduction effect at the time of SCM estimation for effective sub carrier in the frequency domain by removing an uncorrelated component existing beyond a maximum delay time of a channel from an SCM estimated by considering merely the time-frequency domain transform and a time-domain correlation characteristic of a channel for the initial SCM as in Formula 7.
                                                        S              ^                        k                          (                              n                ,                m                            )                                =                                    F              ⁢                                                          ⁢              F              ⁢                                                          ⁢                                                T                  K                                ⁡                                  [                                                            s                      ^                                        i                                          (                                              n                        ,                        m                                            )                                                        ]                                                      =                          F              ⁢                                                          ⁢              F              ⁢                                                          ⁢                                                T                  K                                ⁡                                  [                                                            d                      i                                        ·                                          s                      i                                              (                                                  n                          ,                          m                                                )                                                                              ]                                                                    ,                                  ⁢                              where            ⁢                                                  ⁢                          d              i                                =                      {                                                                                1                    ,                                                                                                                                    i                                                              ≤                    L                                                                                                                    0                    ,                                                                                        otherwise                    ,                                                                                                          Formula        ⁢                                  ⁢        7            where L represents a low-pass type weighting tap and is set to detect an effective component on a time-domain component of the initial SCM. L is defined as L≧τmax in view of the fact that the time-domain correlation characteristic does not exceed a maximum delay time τmax due to influence of multi-path delay of a channel.
However, when the LPS technique is independently applied to each of elements of the initial SCM as in Formula 7, the Hermitian and positive definite structure of the SCM is destroyed, and thus the interference mitigation performance degrades. In order to prevent the performance degradation, before the LPS technique is applied, a decomposition process needs to be performed on the initial SCM, and existing reference literatures and inventions have focused on studies on the decomposition technique.
Examples of the decomposition technique used before the LPS technique is applied include a variance-correlation decomposition, a spectral decomposition singular value decomposition (SVD), and a Cholesky decomposition.
Of these, studies on the Cholesky decomposition capable of maintaining, in particular, the Hermitian and positive definite structure of the estimated SCM have been actively conducted.
For example, Formula 8 represents an example of the Cholesky decomposition for an initial SCM Rk,jπ of a 2Tx-2Rx MIMO system. After the LPS technique is applied to each of elements of a U matrix (a square-root of a matrix Rk,jπ) that has been subjected to the Cholesky decomposition, a process of regenerating an SCM by Cholesky reconstruction of (Û)H(Û) is necessary.
                                                                                          R                                      k                    ,                    j                                    II                                =                                ⁢                                  [                                                                                                              R                                                      k                            ,                            j                                                                                II                            ,                                                          (                                                              1                                ,                                1                                                            )                                                                                                                                                                            R                                                      k                            ,                            j                                                                                II                            ,                                                          (                                                              1                                ,                                2                                                            )                                                                                                                                                                                                                    R                                                      k                            ,                            j                                                                                II                            ,                                                          (                                                              2                                ,                                1                                                            )                                                                                                                                                                            R                                                      k                            ,                            j                                                                                II                            ,                                                          (                                                              2                                ,                                2                                                            )                                                                                                                                                            ]                                                                                                        =                                ⁢                                                                            (                      U                      )                                        H                                    ⁢                                      (                    U                    )                                                                                                                          =                                ⁢                                                      [                                                                                                                        A                            *                                                                                                    0                                                                                                                                                  B                            *                                                                                                    C                                                                                      ]                                    ⁡                                      [                                                                                            A                                                                          B                                                                                                                      0                                                                          C                                                                                      ]                                                                                      ⁢                                  ⁢                  where          ,                                          ⁢                      {                                                                                A                    =                                                                                            (                                                      R                                                          k                              ,                              j                                                                                      II                              ,                                                              (                                                                  1                                  ,                                  1                                                                )                                                                                                              )                                                *                                            ⁢                                              (                                                  R                                                      k                            ,                            j                                                                                II                            ,                                                          (                                                              1                                ,                                1                                                            )                                                                                                      )                                                                                                                                                              B                    =                                                                  R                                                  k                          ,                          j                                                                          II                          ,                                                      (                                                          1                              ,                              2                                                        )                                                                                              /                                                                        (                                                      R                                                          k                              ,                              j                                                                                      II                              ,                                                              (                                                                  1                                  ,                                  1                                                                )                                                                                                              )                                                *                                                                                                                                                              C                    =                                                                                            R                                                      k                            ,                            j                                                                                II                            ,                                                          (                                                              2                                ,                                2                                                            )                                                                                                      -                                                                              B                            *                                                    ⁢                          B                                                                                                                                                                            Formula        ⁢                                  ⁢        8            
Referring back to FIG. 1, an LPS-based SCM estimation unit 122 installed in the SCM estimation unit 120 includes a Cholesky decomposition unit 122-1 that performs the Cholesky decomposition defined in Formula 8 on the initial SCM received from the initial SCM estimation unit 121, an LPS executing unit 122-2 that applies the LPS technique to each of elements of the U matrix that has been subjected to the Cholesky decomposition, and a Cholesky reconstruction unit 122-3 that performs the Cholesky reconstruction after the LPS technique is applied.
The SCM finally estimated by the SCM estimation unit 120 is input to a data symbol demodulation unit 130. The data symbol demodulation unit 130 demodulates a data symbol with mitigated inter-cell interference using the SCM and the channel estimation result.
Generally, in the OFDMA communication system, in order to reduce interference from an adjacent channel, a guard band (GB) with a zero value, in which information is not transmitted, is allocated to a sub carrier of a high frequency band and a sub carrier of a low frequency band, respectively.
Due to setting of the guard band, only the initial SCM in an effective sub carrier zone can be estimated, and thus Formula 4 can be represented again by Formula 9 and Formula 10.
                                          S            k                          ′              ⁡                              (                                  n                  ,                  m                                )                                              =                                    S              k                              (                                  n                  ,                  m                                )                                      ·                          B              k                                      ,                              where            ⁢                                                  ⁢                          B              k                                =                      {                                                                                1                    ,                                                                                                                                    k                                                              ≤                                                                  K                        u                                            2                                                                                                                                        0                    ,                                                                    otherwise                                                                                        Formula        ⁢                                  ⁢        9                                          s          i                      ′            ⁡                          (                              n                ,                m                            )                                      =                              I            ⁢                                                  ⁢            F            ⁢                                                  ⁢            F            ⁢                                                  ⁢                                          T                K                            ⁡                              [                                  S                  k                                      ′                    ⁡                                          (                                              n                        ,                        m                                            )                                                                      ]                                              =                                    s              i                              (                                  n                  ,                  m                                )                                      ⊗                          b              i                                                          Formula        ⁢                                  ⁢        10            
Here, Ku represents the number of effective sub carriers, and Bk represents an ideal band pass function. At this time, bi is a K-point inverse fast Fourier transform (IFFT) result of Bk and causes a spectral leakage represented by Formula 11. That is, an abrupt change in a signal at a border point between an effective sub carrier zone and a guard band zone induces a spectral leakage of a time-domain SCM.
                              b          i                =                              I            ⁢                                                  ⁢            F            ⁢                                                  ⁢            F            ⁢                                                  ⁢                                          T                K                            ⁡                              [                                  B                  K                                ]                                              =                                                    K                u                            K                        ⁢                                          sin                ⁡                                  (                                      πⅈ                    ⁢                                                                                  ⁢                                                                  K                        u                                            /                      K                                                        )                                                            πⅈ                ⁢                                                                  ⁢                                                      K                    u                                    /                  K                                                                                        Formula        ⁢                                  ⁢        11            
FIGS. 2A and 2B are conceptual diagrams for explaining the spectral leakage of the time-domain SCM caused due to the presence of the guard band.
Due to the spectral leakage, spectral leakage components 210 appear in an area beyond a maximum delay time, that is, spectral leakage components 210 appear in all samples in the time-domain (see FIG. 2A).
In particular, when a high-order modulation scheme such as an M-ary quadrature amplitude modulation (QAM) is applied, a spectral leakage phenomenon 220 gets deepened (see FIG. 2B), and thus a serious SCM estimation error is generated in the conventional LPS technique.
As described above, the LPS technique used as the SCM estimation method for inter-cell interference mitigation in the MIMO-OFDMA communication system has a problem in that a complicated decomposition process such as the Cholesky decomposition and the reconstruction process are necessary, and the SCM estimation error is generated since the spectral leakage caused by an abrupt change in a signal at the border point between the effective sub carrier zone and the guard band zone is not considered.